The present invention relates to an image transmission apparatus for transmitting image data and, in particular, to an image transmission apparatus which can prevent deteriorations in image quality even if some transmission data is lost in a transmission line.
Since an image is composed of a number of pixels arranged two-dimensionally, image information is sent through a transmission line in the following manner, on the transmission side, a prescribed number of pixels are picked up from the two-dimensionally arranged pixels and combined with transmission control data, to thereby form a frame. The image information is transmitted on a frame-by-frame basis. On the receiving side, the transmitted pixels are arranged at their original two-dimension positions, to thereby reconstruct the image. In the above image transmission scheme, there may be used a transmission line which has a possibility that some transmission data is lost during transmission. A lack of some transmission data causes adverse influences on the reconstructed image. This will be explained in detail with reference to FIG. 10.
In the case of sending an image as shown in part (a) of FIG. 10 after dividing it into a number of unit images i.sub.1, i.sub.2, i.sub.3, . . . each consisting of a prescribed number of pixels (see part (b)), transmission control data is added to the prescribed number of pixels, to form a frame, as shown in part (c). The image information is transmitted on a frame-by-frame basis.
If a frame is lost during transmission as shown in part (d) of FIG. 10, the unit images that should be received in the order of i.sub.1, i.sub.2, i.sub.3, . . . are actually received in the order of i.sub.1, i.sub.2, i.sub.4, . . . as shown in part (e). As a result, pixel positions deviate as shown in part (f); that is, the original image of part (a) is not reconstructed properly. Some measure is needed against a lack of transmission data.
The following schemes are known as methods for preventing deviations of pixel positions due to a lack of transmission data and enabling proper image reconstruction.
Conventional Scheme 1
For each frame, a response is made to inform the transmission side whether the frame has been correctly received or not. If the frame is found lost, the same frame is transferred again.
Conventional Scheme 2
An address representing a two-dimensional position is added to each set of pixels of a prescribed number. An image is reconstructed by arranging the sets of pixels at the positions represented by the respective addresses (for example, see Japanese Unexamined Patent Publication No. Hei. 4-239840).
Referring to FIG. 10, according to conventional method 1, after transmitting an unit image i.sub.1, the transmission side waits for a response from the receiving side indicating whether the unit image i.sub.1 has been received correctly or not. When the response indicates correct reception of the unit image i.sub.1, the transmission side transmits the next unit image i.sub.2. However, when the response indicates that the unit image i.sub.1 has not been received correctly, the transmission side again transmits the unit image i.sub.1. Repetition of the above operation prevents deviations of pixel positions due to a lack of a frame.
According to conventional method 2, addresses (1, 1), (1, 2), . . . representing positions are added to the respective unit images i.sub.1, i.sub.2, . . . as shown in part (g) of FIG. 10, and resulting frames are transmitted. In the receiving side, the unit images i.sub.1, i.sub.2, . . . are arranged at the positions represented by the respective addresses. Therefore, as shown in part (h), the pixel positions do not deviate even if a frame is lost.
As described above, where a transmission line is used in image transmission which line has a possibility that some transmission data is lost during transmission, a certain measure is needed against a lack of image data. However, the conventional schemes have the three problems described below.
Problem (1): In conventional method 1, the effective bandwidth of data transmission is lowered, because a response is made from the receiving side and possibly a missing frame is retransmitted on a frame-by-frame basis.
Problem (2): In conventional method 2, although no response is needed from the receiving side, a missing frame causes a visual interference in which a portion of the received image information becomes ineffective in a concentrated manner as shown in part (h) of FIG. 10.
Problem (3): With regard to problem (2), Japanese Unexamined Patent Publication No. Hei. 4-239840 (mentioned above) proposes a means for avoiding the above visual interference in which when a plurality of blocks each including a prescribed number of pixels are converted to frames, rearrangement is so made that image data of the originally adjacent blocks do not adjoin each other. However, in the proposed means, since each of blocks included in one frame has an address, the effective bandwidth of the image data transmission is lowered. To reduce the quantity of ineffective image data, it is necessary to reduce the block size. However, if the block size is reduced, the number of addresses representing positions of the respective blocks is increased, resulting in a lowered effective bandwidth of the image data transmission.